To prevent fuel vapors from the fuel tanks of motor vehicles whose internal combustion engines are operated with gasoline from escaping into the environment, in most countries tank ventilation systems are mandated for those vehicles with which the fuel tank is vented and the fuel vapors from the fuel tank are supplied to the intake manifold of the internal combustion engine for combustion in it. Tank ventilation systems generally comprise a fuel vapor reservoir in the form of an activated charcoal-filled reservoir tank which communicates with the fuel tank, through which air from the exterior can be intaken into the intake manifold of the internal combustion engine for regeneration of the activated charcoal. To initiate regeneration, a normally closed regeneration valve which is conventionally referred to as a tank venting valve in the connecting line between the fuel vapor reservoir and the intake manifold is opened. Since, in the case of a defect or problem of the tank venting valve, regeneration of the activated charcoal is not possible, proper operation of the tank venting valve must be regularly checked in order to detect a defect or problem early on and to prevent escape of fuel vapors into the environment by replacing the valve.
Methods for checking the function of a tank venting valve are disclosed, for example, in DE 100 43 071 A1, DE 103 24 813 A1, DE 10 2005 049 068 A1 and DE 10 2006 034 807 A1. In the method of the initially known type disclosed in DE 103 24 813 A1, the tank venting valve in the operating state of the internal combustion engine is repeatedly opened in order to supply to the internal combustion engine the stored fuel vapor from the fuel vapor reservoir and to detect the reaction of the fuel/air ratio control circuit to the opening of the tank venting valve in order to deduce therefrom the function of the tank venting valve.
As in the method described in DE 103 24 813 A1, the quantity which is dependent on the opening state of the tank venting valve is often the fuel/air ratio in the exhaust gas flow of the internal combustion engine which is measured and evaluated by means of a lambda probe. Since additional fuel/air mixture is delivered into the intake manifold and thus to combustion when the tank venting valve has been opened, the λ value in the exhaust gas flow briefly changes.
In addition to the fuel-air ratio in the exhaust gas flow, however, other system or controller variables can also be monitored, such as, for example, the change of the induction pipe pressure in the intake manifold of the internal combustion engine when the tank venting valve is opened or closed, or the change of the energy flow via the throttle valve according to DE 100 43 071 A1, this energy flow being the product of the air flowing through the throttle valve and the efficiency with which this air is burned after mixing with fuel.
The changes are usually compared to a threshold value, proper operation of the tank venting valve being deduced when the change exceeds a threshold value, while a defect or malfunction is assumed when the change does not exceed the threshold value.
The function check of the tank venting valve is generally done when the internal combustion engine is idling, where constant operating conditions prevail over a longer time interval; this facilitates evaluation of the quantity which is to be monitored. But the function check can also be done according to DE 10 2005 049 068 A1 during active tank ventilation operation or according to DE 103 24 813 A1 under load, in the latter case operating states with a low load being preferred since changes of the operating condition take place less dynamically there.
Depending on the load state of the internal combustion engine and the quantity to be monitored, its change will follow opening of the tank venting valve with a more or less large time shift.
It is common to the known methods that the quantity to be monitored, such as, for example, the fuel-air ratio in the exhaust gas flow or the induction pipe pressure, can have a very small amplitude; in conjunction with the time shift between the opening of the tank venting valve and the change of the quantity to be monitored this can make the detection of the latter much more difficult or even impossible.
On this basis, the object of the invention is to improve a method of the initially named type such that even in the case of very small and/or time-shifted amplitudes of the quantity to be monitored, a reliable function check of the tank venting valve is possible.